Antigen specific immune response is concerned with the recognition and ultimate disposal of the antigen/immunogen in a highly discriminatory fashion. Specific immune responses are mediated through two types of effectors mechanisms. One is mediated by antibody produced by lymphocytes (humoral response) and the other is mediated by specially sensitized lymphocytes themselves (cell mediated immunity). The humoral responses are mainly responsible for providing prophylaxis against disease (Prophylactic vaccine). Prophylactic vaccines are administered in anticipation of a disease. Cell mediated immune responses are characterized by release of cytokine from immunized cells on exposure to an antigen. Cell mediated immunity is desirable for management of active disease (Therapeutic vaccine). Therapeutic vaccines are administered in presence of an active disease. The vaccine includes antigen(s) in a pharmaceutically acceptable carrier.
Antigens include immunogens, allergens. Antigens can be pathogens or varieties of material derived from pathogens like virus, bacteria, fungi, parasites. Tumor cells, mammalian cells and material derived from them can also be used as an antigen. The cells and organisms like virus, bacteria are used in the intact form e.g. Polio, BCG, Rabies etc. Chemical composition of antigen is widely variable and include peptides (of various kinds like plain peptide, poly peptides, Lipopetides etc), polysaccharides, polysaccharide congugates, lipids, glycolipids, carbohydrates, proteins, nucleic acids or antigen encoded into nucleic acids.
Antigens are categorized in varieties of ways. Some of them are described below.
Immunogen—The substance that provokes the immune response when introduced into the body. An immunogen is always a macromolecule (protein, polysaccharide). Its ability to stimulate the immune reaction depends on its commonness to the host, molecular size, chemical composition and heterogeneity (e.g. similar to amino acids in a protein).
Allergen—An allergen is a substance that causes the allergic reaction. It can be ingested, inhaled, injected or comes into contact with skin.
(II) Antigens can be Classified in Order of their Origins.
Exogenous antigens—Exogenous antigens are antigens that have entered the body from the outside, for example by inhalation, ingestion, or injection.
Endogenous antigens—Endogenous antigens are antigens that have been generated within the cell, as a result of normal cell metabolism, or because of viral or intracellular bacterial infection. The fragments are then presented on the cell surface in the complex with class I histocompatibility molecules.
(III) Types of Antigen
Tumor antigens—Tumor antigens are those antigens that are present on the surface of tumor cells. These antigens can sometimes be presented only by tumor cells and never by the normal ones. In this case, they are called tumor-specific antigens and typically result from a tumor specific mutation. More common are antigens that are presented by tumor cells and normal cells, and they are called tumor-associated antigens. Cytotoxic T lymphocytes that recognize these antigens may be able to destroy the tumor cells before they proliferate or metastasize. Tumor antigens can also be on the surface of the tumor in the form of, for example, a mutated receptor, in which case they will be recognized by B cells.
Pathogen associated antigens—Antigens are derived from pathogens like virus, Bacteria, fungus, parasites e.g. rabies, Hepatitis B, mump, measles, tetanus, diphtheria etc.
Production of Antigen
Antigens can be produced by recombinant technologies, extraction methods, chemical synthesis, fermentation etc. It can be in the form of a compound or an organism which is natural or genetically modified or a fraction of an organism, which is naturally occurring or genetically modified. Nucleic acids are increasingly being developed and identified as antigens, as in DNA vaccines. Antigens can be administered in the form of naked antigens or encapsulated, coated form, conjugated, mixed, coupled and/or formulated with adjuvant.
Most of the vaccines when applied alone does not produce an adequate immune stimulus. Adjuvants are added to antigen in vaccine composition to enhance the body's immune response to specific antigens of the vaccine. Adjuvants are not necessarily immunogenic themselves. Adjuvants may act by retaining the antigen locally near the site of administration to produce a depot effect facilitating a slow, sustained release of antigen to cells of the immune system. Adjuvants can also attract cells of the immune system to an antigen depot and stimulate such cells to elicit immune responses. Adjuvant can also act by activating antigen presenting cells.
Adjuvants are used to,
Increase antibody response in magnitude or function (e.g. avidity)
Cell mediated immune response
Reduction in antigen dose
Time of mounting response (Faster response, sustained response etc.)
Induction of mucosal immunity
Improve seroconversion and/or seroprotection rates.
One of the approved adjuvants for human use is alum. The adjuvant activity of alum was first discovered in 1926 by Glenny (Chemistry and Industry, Jun. 15, 1926; J. Path. Bacterial, 34, 267). Aluminum salts (alum) have been useful for some vaccines like hepatitis B, diphtheria, tetanus, toxoid etc., but not useful for others like rabies MMR, typhoid etc. It fails to induce cell-mediated immunity. Aluminum hydroxide and aluminum phosphate is collectively commonly referred to as alum. Reports indicate that alum failed to improve the effectiveness of whooping cough and typhoid vaccines and provided only a slight effect with adenovirus vaccines.
A wide range of other materials are also known to have adjuvant activity provokes potent immune responses to antigens. These includes but not limited to,
Saponins like QS21, ISCOMS
Saponins complexed to membrane protein antigens (immune stimulating complexes),
Pluronic polymers with mineral oil,
Killed Mycobacteria in mineral oil, a water-in-mineral-oil emulsion which contains killed/dried mycobacteria in the oil phase, a weaker formulation without the mycobacteria,
Freund's complete adjuvant,
Freund's incomplete adjuvant,
Bacterial products, such as muramyl dipeptide (MDP) and lipopolysaccharide (LPS), MPL as well as lipid A,
Liposomes, a membrane active glucoside extracted from the tree Quillia saponaria, nonionic block copolymer surfactants,
Non metabolised synthetic molecules which tend to bind proteins to cell surfaces; ISCOMS,
Infectious particles
Oil-in-water emulsions—MF59
CpG (Oligonucleotides)—TLR agonists
Other TLR agonists like imiquimod
Immunopeptides
Complete Freund's adjuvant (CFA) is a powerful immunostimulatory agent that has been successfully used with many antigens on an experimental basis. CFA includes three components: a mineral oil, an emulsifying agent, and killed Mycobacterium tuberculosis. Aqueous antigen solutions are mixed with these components to create a water-in-oil emulsion. Although effective as adjuvant, CFA causes severe side effects e.g. pain, abscess formation, fever etc. CFA, therefore, is not used in preparation of commercial vaccines.
Incomplete Freund's adjuvant (IFA) is similar to CFA but does not include the bacterial component. It is a oil in water emulsion. However, evidence indicates that both the oil and emulsifier used in IFA can cause tumors in mice. Muramyl dipeptide (MDP) has been found to be the minimal unit of the mycobacterial cell wall complex that generates the adjuvant activity observed with CFA. e.g., Ellouz et al., Biochem. Biophys. Res. Commun. (1974) 59:1317.
Several synthetic analogs of MDP have been generated that exhibit a wide range of adjuvant potency and side effects (Chedid et al., Prog. Allergy (1978) 25:63). Representative analogs of MDP include threonyl derivatives of MDP (Byars et al., Vaccine (1987) 5:223), n-butyl derivatives of MDP (Chedid et al., Infect. Imrriun. 35:417), and a lipophilic derivative of a muramyl tripeptide (Gisler et al., in Immunomodulations of Microbial Products and Related Synthetic Compounds (1981) Y. Yamamura and S. Kotani, eds., Excerpta Medica, Amsterdam, p. 167). One lipophilic derivative of MDP is N-acetylmurarnyl-L-alanyl-D-isogluatminyl-L-alanine-2-(1′-2′dipalmitoyl-sn-glycero-3-hydroxyphosphoryloxy)-ethylamine (MTP-PE). The MTP-PE itself is able to act as an emulsifying agent to generate stable oil-in-water emulsions. MTP-PE has been used in an emulsion of squalene with TWEEN 80, termed MTP-PE-LO (low oil), to deliver the herpes simplex virus gD antigen with effective results (Sanchez-Pescador et al., J. Immunol. (1988) 141:1720-1727), albeit poor physical stability.
Synthetic polymers are evaluated as adjuvants. These include the homo- and copolymers of lactic and glycolic acid, which have been used to produce micro-spheres that encapsulate antigens (see Eldridge et al., MoI. Immunol. 28:287-294 (1993)).
Nonionic block copolymers are another synthetic adjuvant being evaluated. Adjuvant effects are investigated for low molecular weight copolymers in oil-based emulsions and for high molecular weight copolymers in aqueous formulations (Todd et al., Vaccine 15:564-570 (1997)).
The adjuvants using whole cells like insect cells (S. frugiperda) U.S. Pat. No. 6,224,882 are known. The insects or the insect cells infected with some of the insect viruses/infectious agent or any other type of infection, also it is not yet possible to identify that which insect/insect cell is infected and which not hence the use of these can result in low production and a possible threat of transmission of disease to human (WHO report January 2005).
In an article published in Vaccine (1999) 17; 2446-2452, Bacillus of Calmette-Gue{acute over (η)}n (BCG) is used as adjuvant to rabies vaccination in mice. The experimental results show no improvement in serum neutralizing antibody titers in-group of mice immunized with BCG as adjuvant compared to plain vaccine.
U.S. Pat. No. 6,355,414 describes accmannan polysaccharide as adjuvant. U.S. Pat. No. 6,306,404 describes adjuvant & vaccine compositions of mono phosphoryl lipid A, sugar and optionally an amine-based surfactant. U.S. Pat. No. 6,231,859 describes saponin combination as adjuvant. Saponin adjuvants have high systemic toxicities, like haemolysis. The U.S. Pat. No. 6,060,068 describes interleakin-2 as adjuvant to vaccines. U.S. Pat. No. 6,355,256 describes QS-21 & IL-12 as adjuvants.
PCT application WO/2006/114680 discloses Mycobacterium w as adjuvant with pharmaceutically acceptable carrier and its uses. U.S. Pat. Nos. 6,103,697, 6,228,373 & 6,228,374 describes peptides as adjuvants. JP 11106351, JP 9268130 & AU 780054 describe oil adjuvants. U.S. Pat. No. 6,383,498 discloses compositions of vaccine wherein Neuraminidase and galactose oxidase together are a vaccine adjuvant. U.S. Pat. No. 7,579,009 discloses an immune modulator composition and/or pharmaceutical composition comprising whole cell of a bacterium. U.S. Pat. No. 6,375,945 discloses adjuvant composition comprising a mixture of a saponin adjuvants and use of there compositions in prophylactic and therapeutic applications, particularly in vaccine including cancer vaccines. U.S. Pat. No. 6,306,404 discloses adjuvant and vaccine composition of monophosphoryl lipid. U.S. Pat. No. 7,488,490 discloses unmethylated CpG dinucleotide (CpG ODN) and a non-nucleic acid adjuvant. But in all these adjuvants are not demonstrated with wide variety of antigens and mammals.
The above mentioned adjuvants are at various stages of development. There is still a need to have novel adjuvants to provide novel vaccine with reduced antigen, reduced frequency, increased immunogenic potential. Currently available adjuvants are outcome of painstaking research in absence of a method to identify potential compound with adjuvant properties. It is long standing need of the industry to provide adjuvants that are free of above-mentioned side effects.